The White House is refusing to allow political adviser David Simas to respond to a subpoena from House Oversight chairman Darrell Issa.

White House counsel W. Neil Eggleston says Issa has no power to compel Simas to testify at a hearing Wednesday morning about whether the office he runs has been engaged in improper political activity in violation of the Hatch Act, which bars federal employees from such activities as campaign fundraising and explicit political support. Eggleston cited a new opinion from the Justice Department’s Office of Legal Counsel that cited precedents going back to Presidents Harry Truman and Richard Nixon of executive privilege being asserted against testimony by White House aides.

Issa responds that in a case brought by congressional Democrats in 2008 against the Bush White House, a federal judge found that the idea of absolute immunity of a White House official from a congressional subpoena was “unprecedented” and held that presidential aides are “not absolutely immune from congressional process.”

“Flouting a federal judge’s opinion about our system of checks and balances is yet another attack on our nation’s Constitution by this president,” Issa said in a statement late Tuesday night. “This hearing seeks to examine a political office embedded within the White House which, under Democratic and Republican administrations, has had a controversial role of coordinating political campaign activity for the president at taxpayer expense.”

Issa has a troubling stack of examples of Hatch Act violations by federal officials during the Obama administration. Most notably, the U.S. Office of Special Counsel found in 2012 that Health and Human Services Secretary Kathleen Sebelius had violated the Hatch Act during a political swing through North Carolina earlier that year.

Another example was revealed this week when it came to light that attorney April Sands, who worked for Lois Lerner at the Federal Election Commission before she moved to run the now-infamous nonprofit division of the IRS, has been forced to leave the government over Hatch Act violations.

Sands admitted to violating the Hatch Act by soliciting political contributions via Twitter and conducting political activity through her Twitter account while on duty. The FEC’s Office of Inspector General wanted to pursue criminal charges against her but was stymied when it was found the FEC had “recycled” Ms. Sands’s hard drive before the OIG was able to seize it. Justice Department officials then declined to prosecute due to a lack of evidence.

Recall that Lerner herself saw her own hard drive damaged while she was at the IRS, with the subsequent loss of two years of e-mails. As a result, it will be much more difficult to establish if Lerner was coordinating her targeting of conservative nonprofit groups with the FEC, Justice Department, or the White House.

Chairman Issa hasn’t revealed what sparked his interest in the activities of the White House’s political office, but his hearing today may provide some clues even if his star witness refuses to show up.